I barely got my head around the inflationary universe, and it's already passe.
Sean Carroll starts us off with his debut article in Discover -- Welcome to the Multiverse. That's just a warmup though, his blog digs a lot deeper. ...
The Eternally Existing, Self-Reproducing, Frequently Puzzling Inflationary Universe | Cosmic Variance | Discover Magazine
... it is crucial to note that in conventional non-inflationary cosmology, our current observable universe was about a centimeter across at the Planck time. That’s a huge size by particle physics standards. In inflation, by contrast, the whole universe could have fit into a Planck volume, 10-33 centimeters across, much tinier indeed...
... “essentially all” — models of inflation lead to the prediction that inflation never completely ends. ... inflation will end in some places, but in other places it keeps going. Where it keeps going, space expands at a fantastic rate. In some parts of that region, inflation eventually ends, but in others it keeps going. And that process continues forever, with some part of the universe perpetually undergoing inflation. That’s how the multiverse gets off the ground — we’re left with a chaotic jumble consisting of numerous “pocket universes” separated by regions of inflating spacetime...
... thinking about black hole entropy has led physicists to propose something called “horizon complementarity” — the idea that one observer can’t sensibly talk about things that are happening outside their horizon. When applied to cosmology, this means we should think locally: talk about one or another pocket universe, but not all of them at the same time. In a very real sense, the implication of complementarity is that things outside our horizon aren’t actually real — all that exists, from our point of view, are degrees of freedom inside the horizon, and on the horizon itself....
Sean ends by sending us back to read a 2007 article than runs through stories of pre-inflationary creation ... How Did the Universe Start? (April 2007)
That's a lot to digest, but the very next day Sean features a rant by Tom Banks, a fierce physicist who must put coffee in his Ritalin ...
Guest Post: Tom Banks Contra Eternal Inflation | Cosmic Variance | Discover Magazine
A lot of research in high energy theory has been devoted to the topic of eternal inflation. More and more, over the last few years, I’ve come to regard this as an enormous waste of intellectual resources and I’ve chosen Cosmic Variance as a very public way to make my objections to this theoretical mistake clear...
... We also discussed a solution to Einstein’s equations which was a black hole with de Sitter interior embedded in this homogeneous isotropic cosmology. In the paper referred to above, we have found an exact quantum model, satisfying all the consistency conditions of HST, which corresponds to that solution. There is a one parameter family of models corresponding to the choice of dS c.c. We can also find approximate solutions of the consistency conditions corresponding to two or more such black holes, separated by a large distance...
... So we can construct models in which there are many values of the c.c. depending on which black hole interior one resides in. Each mini dS universe will be stable, unless it collides with another...
I didn't get much out of Tom Banks essay -- it's aimed at someone who knows something. I'm left with a vague sense that we're on the wrong side of a black hole and we know it as our universe. Of course since we have black holes in our universe, it's presumably holes all the way down. Maybe we're computational ghosts replaying whatever fell in from the other side.
Fortunately I've read Greg Egan's Permutation City so this feels pretty comfortable.
Carroll thinks we'll get this figured out sometime in the next 30 years or so. I hope I live to see it.
Setting aside the "simulation" thesis for the moment, and inspired by Egan but unconstrained by knowledge or data, I'll make a guess as to how it will all turn out. For the fun of it, because, after all, this is my blog.
I think the infinities will go away and that everything everywhere will all sum to zero.
I think when this theory is explained to someone like me, physicists will use the analogy of a granite cube 3 meters on a side. They'll say that in this cube is every shape and form there could ever be, all atop one another, waiting to be revealed by the sculptor.
Or they'll show how two sounds can produce silence, and tell us that in silence is every sound, every word, every signal and thought that could ever be. All at once.
